"Grandeur and novelty fix the attention for a considerable time, excluding all other ideas; and the mind thus occupied feels no vacuity."

Metaphor in Context

The natural rate of succession, depends also in some degree upon the particular perceptions that compose the train. An agreeable object, taking a strong hold of the mind, occasions a slower succession than when the objects are indifferent. Grandeur and novelty fix the attention for a considerable time, excluding all other ideas; and the mind thus occupied feels no vacuity. Some emotions, by hurrying the mind from object to object, accelerate the succession. Where the train is composed of connected objects, the succession is quick. For it is so ordered by nature, that the mind goes easily and sweetly along connected objects*. On the other hand, the succession must be slow where the train is composed of unconnected objects. An unconnected object, finding no ready access to the mind, requires time to make an impression. And that it is not admitted without a struggle, appears from the unsettled state of the mind for some moments after it is presented, wavering betwixt it and the former train. During this short period, one or other of the former objects will intrude, perhaps oftener than once, till the attention be fixt entirely upon the new object. The same observations are applicable to ideas suggested by language. The mind can bear a quick succession of related ideas. But an unrelated idea, for which the mind is not prepared, takes time to make a distinct impression; and therefore a train composed of such ideas, ought to proceed with a slow pace. Hence an epic poem, a play, or any story connected in all its parts, may be perused in a shorter time, than a book of maxims or apothegms, of which a quick succession creates both confusion and fatigue.
(I.ix, pp. 383-5)